Traditional image capture involves the concatenation of two reproduction processes, photography and scanning. Unfortunately, traditional photography or the use of digital cameras using three channels is inherently device metameric and cannot reproduce the colors of the original object under different illuminants and for different observers. As a consequence, large color distortions can result during the image recording process. In other words, the colors of reproduced images may not accurately match the colors of the original scene or object. Furthermore, the variance in match equality due to device metamerism can be large, resulting in a dramatic reduction in color quality. Consequently, unless the color components in a reproduced image spectrally match those of the original image, metamerism will occur resulting in an observable difference between the original and the reproduced image under different light
Metamerism is a situation in which spectrally different color stimuli look alike to a human because they have the same tristimulus values. Metamerism also defines apparent changes in color in a reproduction as compared to the original, as seen by an observer under different types of illumination, e.g., daylight vs. incandescent light, or by different observers under a single illumination. The reason for metamerism is that the observed color is a product of the color in the image and the color of the illumination, different color components in an image reflect the light differently. Device metamerism is a situation in which spectrally different color stimuli are represented with identical records by an image capture system.
Image editing is used to correct these observable differences between the original and the reproduced image. Unfortunately, editing not only consumes time and resources, but it is limited in how far it corrects the inherent limitations in color photography. Although the images can be pleasing, they are often unacceptable in terms of color accuracy. As a result, it is impossible to accurately capture original objects using the conventional techniques of photography and scanning.
To ensure that a color match occurs for all observers and under all lighting conditions a spectral match must be achieved. As a result, an image acquisition system should be able to provide signals that lead to a multi-spectral description of a scene. To obtain a multi-spectral description of a scene a variety of different multi-spectral imaging methods and systems have been disclosed, such as in: U.S. Pat. No. 3,684,824; U.S. Pat. No. 3,714,875; U.S. Pat. No. 3,720,146; U.S. Pat. No. 3,758,707; U.S. Pat. No. 4,134,683; U.S. Pat. No. 4,866,454; U.S. Pat. No. 5,109,276; U.S. Pat. No. 5,889,554; and U.S. Pat. No. 5,900,942 which are all herein incorporated by reference. Many of these systems rely upon multi-spectral image acquisition where light reaching a monochrome digital camera is attenuated by a set of narrow-band interference filters which are shifted in series in front of the camera lens.
Unfortunately, when using interference filters for image acquisition a major problem is caused by the transmittance characteristic of the filters which is dependent upon the angle of incidence. For example, in order to image a painting with horizontal dimensions of one meter from a distance of two meters between the painting and the filter, there is angle of incidence 14° for points in the extremities. Simulations have shown that this causes color differences of 2ΔE*ab units in relation to the image obtained at 0° angle of incidence.
Another problem of using interference filters in image acquisition is that the surfaces of the interference filters are not exactly coplanar which results in spatial shifts and distortions within the captured image. Further, there may be inter-reflections caused by light bouncing between the interference filters and the camera lens. As a result, these technical problems have prevented the realization of practical multi-spectral imaging using interference filters without a considerable degree of expertise in multi-spectral imaging as well as complex image processing.